1. Field of the Invention
The present invention relates to an antenna apparatus with directivity switching capability used for a communication terminal apparatus and base station apparatus, etc. in a radio communication system.
2. Description of the Related Art
In radio communications, it is desirable to radiate electromagnetic waves focused on a specific direction and one of the antennas that realize this objective is Yagi antenna. The Yagi antenna is an antenna that controls directivity (radiation direction) by means of the length of a conductor bar placed near a xc2xd wavelength dipole antenna.
This antenna utilizes the nature of radiation direction that inclines toward a parasitic conductor bar placed near an antenna element, which acts as a radiator, if this conductor bar is shorter than xc2xd wavelength, and inclines toward the opposite direction of the conductor bar if the conductor bar is longer than xc2xd wavelength.
Hereafter, an antenna element with directivity toward itself is called xe2x80x9cdirectorxe2x80x9d and an antenna element with directivity toward its opposite direction is called xe2x80x9creflectorxe2x80x9d. The measure used to indicate the sharpness of directivity is called xe2x80x9cgainxe2x80x9d.
Here, in radio communications, there are cases where it is necessary to switch directivity, for example, to minimize a multipath phenomenon that the radio traveling direction varies depending on the transmission environment. As the apparatus with directivity switching capability, the one using an array of a plurality of Yagi antennas made up of 3 elements of reflector, radiator and director is already proposed.
Here, it is possible to achieve higher gain by forming directivity by setting the director and reflector at symmetric positions with respect to the radiator rather than forming directivity using either one of the director or reflector.
FIG. 1A and FIG. 1B show a configuration of a conventional antenna apparatus whose directivity can be changed by 90 degrees.
As shown in FIG. 1A and FIG. 1B, the conventional antenna apparatus consists of basic plate 1, 4 arrays of 3 elements of reflector 2, radiator 3 and director 4 placed in xc2xc wavelength intervals on basic plate 1 and distributed in 90-degree intervals on the horizontal plane, switch circuit 4 inserted into the output of radiator 3 of each antenna array and switching circuit 5 that switches connection/disconnection of switch circuit 4. The reason that the antenna elements are placed in xc2xc wavelength intervals is that the antenna element interval smaller that this would reduce impedance due to mutual coupling.
The conventional antenna apparatus above implements switching of directivity by 90 degrees by changing switching circuit 5 as shown in the directivity diagram in FIG. 2.
However, the conventional antenna apparatus requires the same number of Yagi antenna arrays with antenna element intervals of approximately xc2xc wavelength, as the number of directivities to be switched, causing a problem of increasing the size of the apparatus.
Furthermore, the conventional antenna apparatus has a switch circuit inserted into each radiator output, which will cause another problem that the antenna gain will be reduced due to loss in those switch circuits.
It is an objective of the present invention to provide a small, high-gain antenna apparatus with directivity switching capability.
The present invention achieves the objective above by placing a first antenna element that transmits/receives electromagnetic waves and a parasitic second antenna element on a basic plate, inserting a switching section between one end of the second antenna element and the basic plate, connecting or disconnecting the switching section and thereby making the second antenna element act as a reflector or director.